The infrared spectrum covers a range of wavelengths longer than the visible wavelengths, but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The infrared wavelengths extend from 0.75 micrometers to 1 millimeter. The function of an infrared detector is to respond to the energy of a wavelength within some particular portion of the infrared region.
All materials generate radiant energy having characteristic wavelengths within the infrared spectrum depending on the temperature of the material. Many current infrared image detection systems incorporate arrays with large numbers of discrete, highly sensitive detector elements, the electrical outputs of which are connected to signal processing circuitry. By analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and track sources of infrared radiation. Though the theoretical performance of such contemporary systems is satisfactory for many applications, it is difficult to construct structures that adequately interface large numbers of detector elements with associated signal processing circuitry in a practical and reliable manner. Consequently, practical applications for contemporary infrared image detector systems have necessitated further advances in the areas of miniaturization of the detector array and accompanying circuitry, of minimization of noise that results in lower sensitivity of the detected signal, of image enhancement and of improvements in the reliability of the image detection system and economical production of detector arrays and the accompanying circuitry.
Contemporary arrays of detectors, useful for some applications, may be sized to include 256 detector elements on a side, or a total of 65,536 detectors, the size of each square detector being approximately 0.009 centimeters on a side, with 0.00127 centimeters spacing between detectors. Such an array would therefore be 2.601 centimeters on a side. Interconnection of such a subarray to processing circuitry would require connecting each of the 65,636 detectors to processing circuitry within a square, a little more than one inch on a side. Each array may, in turn, be joined to other arrays to form an extended array that connects to 25,000,000 detectors or more. As would be expected, considerable difficulties are presented in electrically connecting the detector elements to associated circuitry and laying out the circuitry in a minimal area. The problems of forming processing circuitry in such a dense environment require minimization of the surface area used for the circuitry.
The outputs of the detector elements typically undergo a series of processing steps in order to permit derivation of the informational content of the detector output signal. The more fundamental processing steps, such as preamplification, tuned band pass filtering, clutter and background rejection, multiplexing and noise suppression, are preferably done at a location adjacent the detector array focal plane. As a consequence of such on-focal plane, or up-front signal processing, reductions in size, power and cost of signal processing can be achieved. Moreover, up-front signal processing helps alleviate performance, reliability and economic problems associated with the construction of millions of closely spaced conductors connecting each detector element to further signal processing networks.
An improved signal-to-noise ratio for a highly sensitive infrared image detector array can be obtained by comparing the signal on a given detector element with the signals on neighboring elements. This comparison can be accomplished by transmitting the raw data from each individual detector element to an on-board data processor where comparison of the signals is made. The transmission of such raw data from the detector element array to the on-board data processor necessitates the transmission of large amounts of data and the use of an extensive amount of electronic data processing circuitry.
As such, although the prior art has recognized the need to perform comparisons among infrared detector outputs, the proposed solutions have to date been ineffective in providing an economical means of signal data processing.